Effect of interplanetary shock waves on turbulence parameters

Abstract. We have performed an extensive statistical investigation of how interplanetary fast forward shocks affect certain turbulence parameters – namely, the normalised cross-helicity (𝜎_𝑐), residual energy (𝜎_𝑟) and magnetic helicity (𝜎_𝑚). A total of 371 shocks detected by Wind at 1 au and seven shocks by Solar Orbiter below 0.5 au have been analysed. We explore how the aforementioned turbulence parameters and their variations across the shock depend on the shock characteristics, i.e. the gas compression ratio, upstream plasma beta, velocity jump and shock angle. We find that in the shock vicinity, fluctuations tend to show outward imbalance (measured by 𝜎_𝑐), dominance of magnetic energy (negative 𝜎_𝑟) and zero 𝜎_𝑚 when averaged over longer periods. The tendency for imbalance and high Alfvénicity (𝜎_𝑟 ∼ 0) increases with increasing shock velocity jump, and decreasing upstream beta and shock angle. Shocks with large velocity jumps and gas compression ratio have considerably more balanced (𝜎_𝑐 ∼ 0) and less Alfvénic fluctuations in their downstream than upstream, presumably resulting both from Alfvénic fluctuations not passing to the downstream and generation of new compressive fluctuations. We also find that frequency of periods fulfilling the criteria for Alfvén fluctuations (AF) usually decreases, while those meeting the criteria for small-scale flux ropes (SFR) increases from upstream to downstream. The occurrence of AF-like periods peaks for quasi-parallel shocks with large velocity jump, and small upstream beta values. The occurrence of SFR in turn increases with increasing gas compression ratio and upstream beta. The shocks observed by Solar Orbiter at 0.3 – 0.5 au feature overall similar distributions of turbulence parameters and similar upstream-to-downstream changes as detected at 1 au. These results are relevant for understanding turbulence and charged-particle acceleration at collisionless shocks.